One difficulty with wireless communications is fading. Fading may result in temporary degradation and/or failure of communications due to the drop in the channel's signal-to-noise ratio (SNR). Slow fading arises when the coherence time of the channel is large relative to the delay constraint of the channel. In this situation, the amplitude and phase change imposed by the channel can be considered roughly constant over a period of use. Slow fading can be caused by events such as shadowing, where a large obstruction such as a hill or large building obscures the main signal path between the transmitter and the receiver.
Thus, there are general needs for systems and methods that are responsive to a slow fading channel. There are also general needs for systems and methods that can maintain a fairly consistent link between a transmitter and receiver in a fading environment. There are also general needs for systems and methods that can exploit a fading channel when channel-conditions are good but can preserve the communication link when channel conditions degrade. There are also general needs for systems and methods that optimize communications over a fading channel while maintaining an average power constraint.